1) cos(aoi) = cos(lat - slp) cos(decA) cos(hrA) + sin(lat - slp) sin(decA)
2) cos(aoi) = cos(zn) cos(slp) + sin(zn) sin(slp) cos(azm - azm_s)

both the azimuth angles (solar and surface) are calculated from north clockwise ranging from (-180 to +180).

a) why these two incidence angle values are different?
b) which one is the correct way to calculate the angle of incidence?